feat: Add "transaction" pointer on Span

[tonyo]

A proposal that we haven't really discussed in details, but was straightforward to implement to showcase. Needed for OTel work (#537).

Problem

Spans know whether they are transactions (e.g. root spans) thanks to isTransaction flag, but there's no nice way to actually "reach" the transaction from a child span. It is needed, for instance, to get a dynamic sampling context for a nested span, in case we want to propagate it to other services.

Proposal

Introduce transaction (name tentative) attribute on Span, that always points to the root span of the local span tree. For actual root spans, transaction is set to nil. With that attribute in place, we can also get rid of isTransaction flag.

Notes and Caveats

1. A side-effect of removing isTransaction and introducing transaction is that naively created Spans will be transactions by default (since they'll have an empty transaction). We don't really promote or support this usecase anywhere (people should be using StartTransaction and StartSpan), but a thing to keep in mind.
2. Alternatively, for transaction spans, transaction can store a pointer to itself, and not nil. A small downside is that it will be impossible to create a root level Span instance in one go: it'll be something like t := Span{..}; t.transaction = &t in tests or elsewhere.

[codecov]

Codecov Report

Base: 76.46% // Head: 76.63% // Increases project coverage by +0.16% 🎉

Coverage data is based on head (f7a6643) compared to base (f39baef).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
+ Coverage   76.46%   76.63%   +0.16%     
==========================================
  Files          30       30              
  Lines        3387     3398      +11     
==========================================
+ Hits         2590     2604      +14     
+ Misses        699      697       -2     
+ Partials       98       97       -1     

Impacted Files	Coverage Δ
tracing.go	88.28% <100.00%> (+0.29%)	⬆️
transport.go	79.21% <0.00%> (+0.84%)	⬆️

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[tonyo]

Ah, scratch that. Just realized we already have the root span info in spanRecorder. Let me think this over again.